Solid golf ball

ABSTRACT

The present invention provides a solid golf ball having excellent rebound characteristics while maintaining good shot feel. The present invention relates to a solid golf ball comprising at least one layer of a core and at least one layer of a cover formed on the core, wherein at least one layer of the core is obtained by vulcanizing and molding a rubber composition, the rubber composition comprises (a) based on 100 parts by weight of a base rubber, (b) from 15 to 50 parts by weight of a metal salt of α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as a co-crosslinking agent, and the metal salt of α,β-unsaturated carboxylic acid has an average particle size of 6 to 30 μm, and (c) 0.1 to 1.0 parts by weight of a vulcanization initiator.

FIELD OF THE INVENTION

[0001] The present invention relates to a solid golf ball. More particularly, it relates to a solid golf ball having excellent rebound characteristics while maintaining good shot feel.

BACKGROUND OF THE INVENTION

[0002] A vulcanized (crosslinked) molded article of rubber composition is typically used for cores of solid golf balls. The rubber composition generally comprises a base rubber, such as polybutadiene; a co-crosslinking agent, such as a metal salt of α,β-unsaturated carboxylic acid; a vulcanization initiator, such as organic peroxide; a filler, such as zinc oxide, barium sulfate; and the like. In the rubber composition for the core, the vulcanization initiator is thermally dissociated to form free radical. It is considered that the free radical attacks the base rubber and co-crosslinking agent, and the co-crosslinking agent is grafted to a main chain of the base rubber and the base rubber is bonded to each other, thereby crosslinking the rubber composition. Therefore, dispersibility and reactivity of the co-crosslinking agent in the rubber composition have great effect on the physical properties of the solid golf balls.

[0003] It has been suggested to improve the dispersibility of the co-crosslinking agent by coating the co-crosslinking agent particle or by reducing an average particle size thereof (Japanese Patent Kokai Publication Nos. 141961/1984, 92781/1985, 196661/1996, 235413/1997, 57068/1999, 57069/1999 and the like).

[0004] Japanese Patent Kokai Publication Nos. 141961/1984 and 92781/1985 suggest that a surface of zinc acrylate particle is coated with a higher fatty acid and a metal salt thereof respectively in order to improve the dispersibility of zinc acrylate in the rubber composition. Rubber composition or a solid golf ball using a metal salt of unsaturated carboxylic acid having an average particle size of not more than 5 μm as the co-crosslinking agent is described in Japanese Patent Kokai Publication Nos. 196661/1996, 235413/1997 and 57068/1999, and rubber composition or a solid golf ball using zinc acrylate having a particle size distribution of 0.1 to 5 μm and an average particle size of 1 to 4.5 μm as the co-crosslinking agent is described in Japanese Patent Kokai Publication No. 57069/1999.

[0005] However, intermolecular bond between the base rubbers each other have greater effect on rebound characteristics of the resulting golf ball than that between the base rubber and co-crosslinking agent. Therefore, the dispersibility is excessively improved by excessively reducing the particle size of the co-crosslinking agent, and the rebound characteristics are not sufficiently obtained. The reason is considered that surface area of the co-crosslinking agent is too large, and the reactivity between the base rubber and co-crosslinking agent is high, which degrades the reactivity between the base rubbers each other.

OBJECTS OF THE INVENTION

[0006] A main object of the present invention is to provide a solid golf ball having excellent rebound characteristics while maintaining good shot feel.

[0007] According to the present invention, the object described above has been accomplished by adjusting the average particle size and amount of the co-crosslinking agent, and the amount of the vulcanization initiator in the rubber composition for the core to specified ranges, thereby providing a solid golf ball having excellent rebound characteristics while maintaining good shot feel.

SUMMARY OF THE INVENTION

[0008] The present invention provides a solid golf ball comprising at least one layer of a core and at least one layer of a cover formed on the core, wherein at least one layer of the core is obtained by vulcanizing and press molding a rubber composition, the rubber composition comprises

[0009] (a) based on 100 parts by weight of a base rubber,

[0010] (b) from 15 to 50 parts by weight of a metal salt of α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as a co-crosslinking agent, and the metal salt of α,β-unsaturated carboxylic acid has an average particle size of 6 to 30 μm, and

[0011] (c) 0.1 to 1.0 parts by weight of a vulcanization initiator.

[0012] The present invention further provides a one piece-solid golf ball obtained by vulcanizing and molding a rubber composition, wherein the rubber composition comprises

[0013] (a) based on 100 parts by weight of a base rubber,

[0014] (b) from 15 to 50 parts by weight of a metal salt of α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as a co-crosslinking agent, and the metal salt of α,β-unsaturated carboxylic acid has an average particle size of 6 to 30 μm, and

[0015] (c) 0.1 to 1.0 parts by weight of a vulcanization initiator.

[0016] The co-crosslinking agent is dispersed in the rubber composition as particle (agglomerate) after mixing, and is melted in the base rubber on vulcanization reaction to be fine dispersed. The co-crosslinking agent is then crosslinking reacted with the base rubber. Since the co-crosslinking agent having very small average particle size is excessively fine dispersed, the reactivity between the base rubber and co-crosslinking agent is too high, which degrades the reactivity between the base rubbers each other. In the present invention, the co-crosslinking agent particle is properly dispersed after mixing the rubber composition by using the co-crosslinking agent having properly large average particle size; and the surface area of the co-crosslinking agent particle is prevented from excessively increasing by maintaining proper particle size thereof. By decreasing the amount of the vulcanization initiator in combination with thereof, the present inventors discovered that the reactivity between the co-crosslinking agent and the base rubber is restrained from excessively increasing, and the reactivity between the base rubbers each other is improved, which improves the rebound characteristics.

[0017] The co-crosslinking agent is reacted in state of particle, and the reaction between the co-crosslinking agent each other also occurs in the particle. Therefore, it is considered that there are co-crosslinking agent particles in the rubber composition after crosslinking. Since the crosslinked co-crosslinking agent particle is generally harder than the base rubber, if the co-crosslinking agent is used in an equal amount of each, the hardness of the resulting core is low with the decrease of the particle size thereof. Therefore, by using the co-crosslinking agent having properly large average particle size, the resulting core and golf ball are soft, and shot feel is improved.

[0018] In the golf ball of the present invention, it is discovered that the reactivity between the co-crosslinking agent and the base rubber is restrained from excessively increasing, and the reactivity between the base rubbers each other, which has great effect on rebound characteristics, is improved, by adjusting the average particle size of the co-crosslinking agent to a proper range and decreasing the amount of the vulcanization initiator as compared with the conventional rubber composition, will result in improving the rebound characteristics.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The solid golf ball of the present invention comprises at least one layer of the core and at least one layer of the cover covering the core. The core is basically obtained by press-molding a rubber composition under applied heat by using a method and condition, which has been conventionally used for preparing solid cores of golf balls. The rubber composition contains a base rubber, a metal salt of α,β-unsaturated carboxylic acid as a co-crosslinking agent, a vulcanization initiator and the like.

[0020] As the base rubber used for the core of the present invention, preferred is high-cis polybutadiene rubber containing a cis-1,4 bond of not less than 40%, preferably not less than 70%, more preferably not less than 90%, because it imparts excellent rebound characteristics to the resulting golf ball. The high-cis polybutadiene rubber may be optionally mixed with natural rubber, polyisoprene rubber, styrene-butadiene rubber, nitrile rubber, ethylene-propylene-diene rubber (EPDM) and the like.

[0021] The co-crosslinking agent can be a metal salt of α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. Examples of the α,β-unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid, fumaric acid and the like. Preferred is acrylic acid, because it imparts high rebound characteristics to the resulting golf ball. Examples of the metal of the metal salt include zinc, sodium, magnesium, calcium, aluminum and the like. Preferred is zinc. Therefore, the preferred co-crosslinking agent is zinc acrylate.

[0022] In the golf ball of the present invention, it is required for the co-crosslinking agent used in the rubber composition for the core to have an average particle size of 6 to 30 μm, preferably 8 to 25 μm, more preferably 10 to 15 μm. When the average particle size is smaller than 6 μm, the dispersibility of the co-crosslinking agent in the rubber composition is too high, and the technical effects of improving the rebound characteristics accomplished by improving the reactivity between the base rubbers each other are not sufficiently obtained. On the other hand, when the average particle size is larger than 30 μm, the co-crosslinking agent particle is too large, and the properties of the resulting golf ball have large variability. The amount of the co-crosslinking agent is from 15 to 50 parts by weight, preferably from 20 to 45 parts by weight, more preferably from 20 to 40 parts by weight, based on 100 parts by weight of the base rubber. When the amount of the co-crosslinking agent is smaller than 15 parts by weight, the core is too soft, and the rebound characteristics are degraded. On the other hand, when the amount of the co-crosslinking agent is larger than 50 parts by weight, the core is too hard, and the shot feel of the resulting golf ball is poor.

[0023] The vulcanization initiator includes organic peroxide such as dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, di-t-butyl peroxide and the like. The preferred organic peroxide is dicumyl peroxide. The organic peroxide is thermally dissociated to form free radical, and the reactivity of crosslinking reaction between the co-crosslinking agent and the base resin is improved.

[0024] In the rubber composition for the core of the present invention, it is required for the amount of the vulcanization initiator to be within the range of from 0.1 to 1.0 parts by weight, preferably 0.2 to 0.8 parts by weight, more preferably 0.3 to 0.8 parts by weight, based on 100 parts by weight of the base rubber. When the amount of the vulcanization initiator is smaller than 0.1 parts by weight, the core is too soft, and the rebound characteristics of the resulting golf ball are degraded. On the other hand, when the amount of the vulcanization initiator is larger than 1.0 parts by weight, the reactivity between the base rubber and co-crosslinking agent is high, and the technical effects accomplished by using the co-crosslinking agent having properly large average particle size are sufficiently obtained.

[0025] It is desired for a volume ratio (V₁/V₂) of a volume (V₁) of rubber layer formed by vulcanizing and press molding the rubber composition comprising the base rubber, co-crosslinking agent and vulcanization initiator (it is total volume of rubber layers when it has multi-layer structure) to a volume (V₂) of the golf ball to be within the range of from 0.3 to 0.9, preferably from 0.5 to 0.85, more preferably from 0.6 to 0.8. When the volume ratio is smaller than 0.3, the technical effects of the present invention of improving the rebound characteristics are sufficiently obtained. On the other hand, when the volume ratio is larger than 0.9, the cover is thin, and the durability of the resulting golf ball is poor.

[0026] The rubber compositions for the core of the golf ball of the present invention can optionally contain filler (such as zinc oxide, barium sulfate, calcium carbonate, magnesium oxide and the like), other components, which have been conventionally used for preparing the core of solid golf balls, such as antioxidant or peptizing agent. If used, the amount of the filler is preferably 5 to 30 parts by weight, and the amount of the antioxidant is preferably 0.2 to 0.5 parts by weight, based on 100 parts by weight of the base rubber.

[0027] The core used in the golf ball of the present invention can be obtained by press-molding and vulcanizing the rubber composition under applied heat in a mold. The vulcanizing is conducted at 130 to 180° C. and 2.8 to 9.8 MPa for 10 to 50 minutes.

[0028] In the golf ball of the present invention, the core has a diameter of 30 to 42 mm, preferably 32 to 40 mm. When the diameter of the core is smaller than 30 mm, the core is too small, and the technical effects accomplished by the presence of the core are not sufficiently obtained. Therefore, the rebound characteristics and durability are degraded. On the other hand, when the diameter of the core is larger than 42 mm, the thickness of the cover is small, and it is difficult to coat the cover.

[0029] In the golf ball of the present invention, it is desired for the core to have a deformation amount when applying from an initial load of 98 N to a final load of 1275 N of 2.0 to 6.0 mm, preferably 2.5 to 5.0 mm, more preferably 2.8 to 4.5 mm. When the deformation amount is smaller than 2.0 mm, the core is too hard, and the shot feel of the resulting golf ball is poor. On the other hand, when the deformation amount is larger than 6.0 mm, the core is too soft, and the durability of the resulting golf ball is poor, or the shot feel is poor.

[0030] The core used in the present invention may have single-layer structure, or multi-layer structure, which has two or more layers. When the core has the multi-layer structure, the rubber composition described above may be used in only one layer or two or more layers, or in any layer. The solid golf ball of the present invention can be a one-piece solid golf ball formed by vulcanizing and press molding only the above rubber composition.

[0031] The cover is then coated on the core. The cover of the present invention is not limited as long as it is formed from cover material, which has been conventionally used for the cover of golf balls, but includes thermoplastic resin, such as ionomer resin, polyethylene resin, polypropylene resin, thermoplastic elastomer and the like, or mixtures thereof. In addition, the cover may include thermosetting polyurethane-based elastomer or balata (trans-polyisoprene).

[0032] The ionomer resin may be a copolymer of ethylene and α,β-unsaturated carboxylic acid, of which a portion of carboxylic acid groups is neutralized with metal ion, or a terpolymer of ethylene, α,β-unsaturated carboxylic acid and α,β-unsaturated carboxylic acid ester, of which a portion of carboxylic acid groups is neutralized with metal ion. Examples of the α,β-unsaturated carboxylic acid in the ionomer include acrylic acid, methacrylic acid, fumaric acid, maleic acid, crotonic acid and the like, preferred are acrylic acid and methacrylic acid. Examples of the α,β-unsaturated carboxylic acid ester in the ionomer include methyl ester, ethyl ester, propyl ester, n-butyl ester and isobutyl ester of acrylic acid, methacrylic acid, fumaric acid, maleic acid, crotonic acid and the like. Preferred are acrylic acid esters and methacrylic acid esters. The metal ion which neutralizes a portion of carboxylic acid groups of the copolymer or terpolymer includes a sodium ion, a potassium ion, a lithium ion, a magnesium ion, a calcium ion, a zinc ion, a barium ion, an aluminum, a tin ion, a zirconium ion, cadmium ion, and the like. Preferred are sodium ions, zinc ions, magnesium ions and the like, in view of rebound characteristics, durability and the like.

[0033] The ionomer resin is not limited, but examples thereof will be shown by a trade name thereof. Examples of the ionomer resins, which are commercially available from Mitsui Du Pont Polychemical Co., Ltd. include Hi-milan 1555, Hi-milan 1557, Hi-milan 1605, Hi-milan 1702, Hi-milan 1705, Hi-milan 1706, Hi-milan 1707, Hi-milan 1855 and the like. Examples of the ionomer resins, which are commercially available from Du Pont Co., include Surlyn 8945, Surlyn 9945, Surlyn 6320, Surlyn 8320, Surlyn 9320 and the like. Examples of the ionomer resins, which are commercially available from Exxon Chemical Co., include Iotek 7010, Iotek 8000 and the like. These ionomer resins may be used alone or in combination.

[0034] Examples of the thermoplastic elastomers include polyamide-based thermoplastic elastomer, which is commercially available from Toray Co., Ltd. under the trade name of “Pebax” (such as “Pebax 2533”); polyester-based thermoplastic elastomer, which is commercially available from Toray-Du Pont Co., Ltd. under the trade name of “Hytrel” (such as “Hytrel 3548”, “Hytrel 4047”); polyurethane-based elastomer, which is commercially available from Takeda Badische Urethane Co., Ltd. under the trade name of “Elastollan” (such as “Elastollan ET880”); polyurethane-based thermoplastic elastomers commercially available from Dainippon Ink & Chemicals Inc., under the trade name “Pandex” (such as “Pandex T-8180”); styrene-based thermoplastic elastomer commercially available from Mitsubishi Chemical Co., Ltd. under the trade name of “Rabalon” (such as “Rabalon SR04”); and the like.

[0035] In the golf ball of the present invention, the cover composition may optionally contain fillers such as barium sulfate, pigments such as titanium dioxide and other additives (such as a dispersant, an antioxidant, a UV absorber, a photostabilizer and a fluorescent agent or a fluorescent brightener, etc.), in addition to the resin component, as long as the addition of the additive does not deteriorate the desired performance of the golf ball cover. If used, the amount of the pigment is preferably 0.01 to 10.0 parts by weight, based on the 100 parts by weight of the base resin of the cover.

[0036] The cover of the present invention may be formed by conventional methods, which have been known to the art and used for forming the cover of the golf balls. When the cover is formed from thermoplastic resin, for example, there can be used a method comprising molding the cover composition into a semi-spherical half-shell in advance, covering the thread wound core with the two half-shells, followed by pressure molding, or a method comprising injection molding the cover composition directly on the thread wound core to cover it.

[0037] In the golf ball of the present invention, it is desired for the cover to have a thickness of 0.5 to 6.0 mm, preferably 1.0 to 4.0 mm, more preferably 1.5 to 3.0 mm. When the thickness of the cover is smaller than 0.5 mm, the cover is too thin, and the durability is degraded. In addition, the rebound characteristics are also degraded. On the other hand, when the thickness is larger than 6.0 mm, the shot feel at the time of hitting is poor.

[0038] At the time of molding the cover, many depressions called “dimples” are formed on the surface of the golf ball. Furthermore, paint finishing or marking with a stamp may be optionally provided after the cover is molded for commercial purposes. The number, shape and size of the dimple are not specially limited. The paint may be a enamel paint of white color or a desired color, or a clear paint. It is well known in the art that paints of the other color may be coated. The paint may optionally contain an antioxidant, a UV absorber, a photostabilizer and a fluorescent agent or a fluorescent brightener, etc.

EXAMPLES

[0039] The following Examples and Comparative Examples further illustrate the present invention in detail but are not to be construed to limit the scope of the present invention.

Preparation of Zinc Acrylate

[0040] A given weight of zinc oxide and triple weight thereof of toluene were charged in a reactor, and then suspended with stirring. Acrylic acid was dropped into the reactor in twice mole ratio of the zinc acrylate to react at the room temperature for 1 hour with stirring. After reacting, the product was heated under a vacuum condition to distill off toluene and formed water, and bulk zinc acrylate was obtained.

[0041] The resulting bulk zinc acrylate was ground using a grinder, and classified by a sieve to obtain four sorts of zinc acrylates I to IV. The average particle size of the resulting four sorts zinc acrylates was determined. The test method is described later. TABLE 1 Average particle size (μm) Zinc acrylate I 2.9 Zinc acrylate II 5.2 Zinc acrylate III 8.6 Zinc acrylate IV 11.4 

Production of Cores

[0042] The rubber compositions for core having the formulation shown in Tables 1 and 2 with the resulting the zinc acrylates I to IV were mixed by using a kneader and mixing roll, and the mixtures were then press molded at 160° C. for 30 minutes in a mold to obtain cores having a diameter of 38.4 mm and a weight of 34.6 g. TABLE 2 (parts by weight) Example No. Com. Ex. No. Core composition 1 2 3 4 1 BR - 01 *1 100 100 100 100 100 Zinc acrylate I — — — — 27 Zinc acrylate II — — — — — Zinc acrylate III 27 36 — — — Zinc acrylate IV — — 27 36 — Zinc oxide 20 17 20 17 20 Dicumyl peroxide 0.6 0.3 0.8 0.4 0.3

[0043] TABLE 3 (parts by weight) Comparative Example No. Core composition 2 3 4 5 6 7 BR-01 *1 100 100 100 100 100 100 Zinc acrylate I — — — — — — Zinc acrylate II 27 — — — — — Zinc acrylate III — 10 55 — — 36 Zinc acrylate IV — — — 10 55 — Zinc oxide 20 26 10 26 10 17 Dicumyl peroxide 0.4 1.5 0.08 1.7 0.08 1.2

Preparation of Cover Compositions

[0044] The formulation materials shown in Table 4 were mixed using a kneading type twin-screw extruder to obtain pelletized cover compositions. The extrusion condition was,

[0045] a screw diameter of 45 mm,

[0046] a screw speed of 200 rpm, and

[0047] a screw L/D of 35.

[0048] The formulation materials were heated at 200 to 260° C. at the die position of the extruder. TABLE 4 Cover composition Amount (parts by weight) Hi-milan 1605 *2 50 Hi-milan 1706 *3 50 Titanium dioxide 2

Examples 1 to 4 and Comparative Examples 1 to 7

[0049] The cover compositions were covered on the resulting core by injection molding to form a cover layer having a thickness of 2.3 mm. Then, clear urethane paint was applied on the surface to produce golf ball having a diameter of 42.7 mm and weight of 45.4 g. With respect to the resulting golf balls, the deformation amount, coefficient of restitution, flight performance (initial velocity, spin amount and shot feel were measured or evaluated. The results are shown in Tables 5 and 6. The test methods are as follows.

Test Method

[0050] (1) Average Particle Size

[0051] The average particle size of the co-crosslinking agent is determined by using a laser diffraction/light scattering particle size distribution analyzer LA-910, available from HORIBA, Ltd.

[0052] (2) Deformation Amount

[0053] The deformation amount of core was determined by measuring a deformation amount when applying from an initial load of 98 N to a final load of 1275 N on the core.

[0054] (3) Coefficient of Restitution

[0055] A aluminum cylinder having a weight of 198.4 g was struck at a speed of 45 cm/sec against a golf ball, and the velocity of the cylinder and the golf ball before and after the strike were measured. The coefficient of restitution of the golf ball was calculated from the velocity and the weight of both the cylinder and the golf ball. The measurement was conducted by using 12 golf balls for each sample (n=12), with the mean value being taken as the coefficient of restitution of each ball and expressed as an index, with the value of the index in Comparative Example 1 being taken as 100. A higher index corresponded to a higher rebound characteristic, and thus a good result.

[0056] (4) Shot Feel

[0057] The shot feel of the golf ball is evaluated by 20 amateur golfers having a handicap of not more than 15 according to a practical hitting test using a No. 1 wood club (W#1, a driver). The results shown in the Tables below are based on the fact that the most golfers evaluated with the same criterion about shot feel. The evaluation criteria are as follows.

[0058] Evaluation criteria I (Impact force)

[0059] ∘: The golfers felt that the golf ball has good shot feel such that impact force at the time of hitting is small.

[0060] Δ: The golfers felt that the golf ball has fairly good shot feel such that impact force at the time of hitting is slightly large.

[0061] x: The golfers felt that the golf ball has poor shot feel such that impact force at the time of hitting is large.

[0062] Evaluation criteria II (Rebound Characteristics)

[0063] ∘: The golfers felt that the golf ball has good shot feel such that rebound characteristics are good.

[0064] Δ: The golfers felt that the golf ball has slightly heavy and fairly good shot feel such that rebound characteristics are small.

[0065] x: The golfers felt that the golf ball has heavy and poor shot feel such that rebound characteristics are poor. TABLE 5 Com. Example No. Ex. No. Test item 1 2 3 4 1 (Core) Deformation amount 3.72 3.56 3.70 3.48 3.75 (mm) (Physical properties of golf ball) Coefficient of restitution 102 103 103 105 100 Shot feel Impact force ∘ ∘ ∘ ∘ ∘ Rebound ∘ ∘ ∘ ∘ Δ

[0066] TABLE 6 Comparative Example No. Test item 2 3 4 5 6 7 (Core) Deformation 3.74 4.21 3.25 4.35 3.38 2.92 amount (mm) (Physical properties of golf ball) Coefficient of 100 95 103 96 104 105 restitution Shot Impact ∘ ∘ x ∘ x x feel force Re- Δ x ∘ x ∘ ∘ bound

[0067] As is apparent from the results shown in Tables 5 and 6, the golf balls of the present invention of Examples 1 to 4 have excellent rebound characteristics and good shot feel when compared with the golf balls of Comparative Examples 1 to 7.

[0068] On the other hand, in the golf balls of Comparative Examples 1 and 2, the average particle size of zinc acrylate is too small, and the dispersibility of the zinc acrylate in the rubber composition is too high. Therefore the technical effects of improving the rebound characteristics accomplished by improving the reactivity between the base rubbers each other are sufficiently obtained, which reduces the coefficient of restitution.

[0069] In the golf balls of Comparative Examples 3 and 5, the amount of the zinc acrylate is too small, and the resulting golf ball is too soft, which reduces the coefficient of restitution. The amount of the dicumyl peroxide is large, and the reactivity between the zinc acrylate and polybutadiene is too high, and the technical effects accomplished by using the co-crosslinking agent having large average particle size are sufficiently obtained, which largely reduces the coefficient of restitution. In addition, the shot feel is heavy and poor such that rebound characteristics are poor.

[0070] In the golf balls of Comparative Examples 4 and 6, the amount of the zinc acrylate is large, and the resulting golf ball is too hard and the shot feel is poor shot feel such that impact force at the time of hitting is large. In the golf ball of Comparative Example 7, the amount of dicumyl peroxide is large, and the resulting golf ball is too hard and the shot feel is poor such that impact force at the time of hitting is large. 

What is claimed is:
 1. A solid golf ball comprising at least one layer of a core and at least one layer of a cover formed on the core, wherein at least one layer of the core is obtained by vulcanizing and molding a rubber composition, the rubber composition comprises (a) based on 100 parts by weight of a base rubber, (b) from 15 to 50 parts by weight of a metal salt of α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as a co-crosslinking agent, and the metal salt of α,β-unsaturated carboxylic acid has an average particle size of 6 to 30 μm, and (c) 0.1 to 1.0 parts by weight of a vulcanization initiator.
 2. The solid golf ball according to claim 1, wherein the co-crosslinking agent is zinc acrylate.
 3. The solid golf ball according to claim 1, wherein the co-crosslinking agent has an average particle size of 8 to 25 μm.
 4. A one piece-solid golf ball obtained by vulcanizing and molding a rubber composition, wherein the rubber composition comprises (a) based on 100 parts by weight of a base rubber, (b) from 15 to 50 parts by weight of a metal salt of α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as a co-crosslinking agent, and the metal salt of α,β-unsaturated carboxylic acid has an average particle size of 6 to 30 μm, and (c) 0.1 to 1.0 parts by weight of a vulcanization initiator. 